The long range objective of this proposal is to determine the detailed structure and the mechanism of assembly of an IgA immunoglobulin secreted by a murine plasmacytoma, MOPC 315. This protein, which is structurally analogous to the human IgA2 (Am plus) subclass, is secreted as a mixture of monomer IgA and of polymeric IgA which contains covalently-linked J chain. It is thus a satisfactory structural model for IgA in general, for a particular IgA subclass and for polymeric IgA. Included are studies directed toward the completion of the primary amino acid sequences of its alpha chain and its associated J chain; and toward the delineation of its intrachain, interchain and, in the case of polymers, intersubunit disulfide bridges. The light chain sequence has been previously determined. Also included are studies directed toward assessing the role of J chain in the assembly of polymeric immunoglobulins. The satisfactory completion of these studies should contribute to understanding the structure of IgA immunoglobulins in general and to the structure of J chain and of its role in the formation of polymeric immunoglobulins.